Spectrophotometric determination of bacterial
growth involves several problems, including
the inability to differentiate dead from living
organisms, variable light scattering by living
and dead organisms, discrepant measurements
with low and high bacterial concentrations and
with aggregating organisms, and interference
with by-products of growth (7, 8). Furthermore,
light transmission and scattering by microorganisms
change between the logarithmic and
stationary phases of growth (8). In our studies,
major limiting factors also included bacterial
aggregation and hemolysis in the blood agar
plates. When aggregation was prominent, (isolates
of B. melaninogenicus subspecies
intermedius and P. assacharolyticus) growth
curves could not be obtained by either the OD
or pour plate methods. Several isolates hemolyzed
the blood agar during growth and the
pigment was recordable; however, the pigment
accounted for an insignificant OD (usually less
than 0.05 U) compared to turbidity changes
secondary to cell multiplication. Curves obtained
by the OD method were also reproducible
when repeated on several occasions. The
growth curves corresponded well to those obtained
by the pour plate method, and the
growth-phase intervals usually (but not always)
corresponded best when the higher inoculum
was used. The prime advantages of the OD
method were that it was technically easier and
required much less time.